Didn't look at your list but I'll chime in. Bought my FG off Craigslist. Found a hobbyist with FG, S&S, EK, DP, Golgar, and HH. I bought the FG with only a players experience. Although I can do most PCB repairs and was trained to solder, doesn't mean I'm the best troubleshooter. I am a prior AGE mechanic with the Air Force so the maintenance skills needed for an 80s Bally are in my favor. Took an expensive gamble and it paid off. It's not museum quality but it plays well and if you know Flash, it's never the same game twice. Except you lose vs. Ming every single time.
It broke a year later. Where I live, I can't take it to someone and no one to come to me. The hobbyist I bought from was done and done with pinball. I'm on my own or using the experts in this community to fix it.
The entire time it was broke I was consumed with it. That's me, and that's just one box. So, I suggest not getting balls deep with two pins as I couldn't imagine trying to deal with two broke pins. And they will break.
If your budget is 9k then get one really grand machine and use the leftovers for upgrades or maintenance. Play the one and let it break on you, then go about trying to fix just the one. I don't recall you stating you knew anything about fixing them, so I'd put the additional money in a high quality soldering station, a DMM, fuses, diodes, and wire. Then learn to use all of it.
I have no maintenance XP with any machine outside FG. So I can't comment on the maintenance of a Stern outside of just playing it.